Determination of the mechanical behavior physical properties of materials is necessary so that materials may be selected for use, evaluated when in use, and evaluated after use. From these determinations, decisions are made as to which materials to use, the conditions under which they can be used, and whether such materials in use can be continued to be used with safety. These types of determinations are particularly useful for determining the effects of environmental loading such as nuclear radiation on the mechanical properties of in-service materials. This invention is fully applicable to the determination of mechanical behavior of such materials but is also applicable to materials not subjected to radiation and the validity of the invention was demonstrated for materials not subjected to radiation.
The prior art includes U.S. Pat. No. 4,567,774 having the inventor, Michael P. Manahan, common with this application, and assigned to the same assignee. This earlier patent, hereinafter referred to as the "prior patent", includes the basic concepts upon which this invention is based. The disclosure of the prior patent is included herein by reference and the portions of that disclosure not specifically needed for the disclosure of this improvement invention are not included herein. However, reference to the prior patent may be helpful to the understanding hereof.
In the past, the most common procedure has been to determine the mechanical behavior of materials by testing large samples that are either created more or less simultaneously or side by side with the product that is intended to be used or are cut from the same batch of material. In the determination of the mechanical behavior of solid materials and particularly metals, the practice is to make tensile, fatigue, creep, stress relaxation, ductile/brittle transition behavior, fracture toughness, etc. specimens; and these are then subjected to loads while measurements are taken of the force, time, displacement, temperature, energy, velocity, crack length, etc. of the specimen. Information on stress and strain which can be thought of as normalized load and deflection respectively as well as other useful parameters are then obtained by simple mathematical operations. For example, in a uniaxial tensile test the stress is determined by dividing the measured load by the specimen cross sectional area.
While this may be satisfactory in most instances, there are other circumstances such as the post-irradiation testing of materials used in nuclear reactors where samples may be unavailable in sufficient size and quantity to carry out these destructive tests during the life of the materials in use. In general, neutron irradiation space for materials investigations is limited and costly. It is, therefore, desirable to use specimens of minimum volume. Since neutron irradiation costs scale with specimen volume, miniaturized mechanical behavior testing can provide significant savings in irradiation testing costs for nuclear materials investigations. In addition, it is possible to provide mechanical behavior information which is not ordinarily obtainable due to space limitations in irradiation experiments and thus expedite alloy development investigations. Of course, miniature specimen testing is applicable to materials investigations for other nuclear technologies as well as non-nuclear technologies requiring mechanical behavior characterizations from a small volume of material. One such non-nuclear application is cutting small pieces of material from in-service components and using miniature specimens to measure the current mechanical behavior state. These data can then be used to estimate the remaining life of the in-service component.
Alternatively, a miniaturized bend test can be employed wherein the fracture mode transition behavior for the material is determined by suspending the specimen between two spaced apart points while simultaneously bringing down a substantially centrally-positioned punch onto the notched and/or precracked specimen to deflect it and by providing other modifications to the specimen to achieve reasonably flat fracture surface and sufficient constraint so that fracture mode transitional behavior can be measured. This type of test can be characterized as a three point bend test. The present invention was conceived as a solution to the problem of determining the fracture mode transition behavior from miniature specimens which are thinner and smaller in volume than those conventionally employed in the art. More particularly, this invention relates to the testing of material specimens of thickness less than thought necessary for valid determinations of ductile-brittle transition temperatures (DBTT) particularly, and less than the minimum size as taught by ASTM A370-77, E23-86, and E8l2-81.
There are four principal conceptual innovative aspects to the miniaturized fracture mode transition behavior (FMTB) testing method of this invention. The first is the use of specimens that are significantly thinner and smaller than those currently in use. The second is the use of an experimental stress field modifying technique so that useful data can be obtained using small specimens. A particular manifestation is application of a transverse load in the thickness direction and/or making side grooves and spacing them such that a nearly constant through thickness stress field of sufficient magnitude so as to yield useful data is achieved. The third is the use of conceived and verified analysis techniques to produce results that have significant, adequate, and useful correlation with the results that are obtained by specified ASTM testing methods such as ASTM E23-86. The fourth is the use of the finite element method to calculate the direction and amount of additional load and/or side grooving to be applied to achieve the desired stress state in the specimen. In a particular manifestation, the finite element code is used to determine the amount of side grooving to achieve sufficient transverse tensile stress such that a reasonably flat fracture surface can be obtained.
This invention improves upon the method of U.S. Pat. No. 4,567,774 by teaching the modifying of the stress field during the testing with the miniature specimen. This modifying of the stress field can be done mechanically, or by using a force field such as a magnetic field in order to produce stresses in preferred orientations in the material, or by causing a change in the stress field conditions by the means of the removal of material on the sides of the specimen. Since plane strain need not be achieved in fracture mode transition behavior testing, side notching of the specimen is the preferred approach since this is experimentally less complicated. The material is removed in the form of a groove or crack on each side of specimen. In essence, the stress field modification replaces the need for material thickness.
In one particularly usefully test configuration, the miniature specimen is loaded in a three point bend test and parameters such as load, deflection, temperature, and time are measured. The data are then analyzed using either fracture appearance as the correlation parameter, or percent post-maximum energy as the new parameter. The data analysis process using this new parameter is necessary in order to obtain conventional full size energy vs temperature Charpy data as described in ASTM E23 using the miniature specimen data.
In essence, the stress field modification replaces the need for material thickness. The stress field is modified by providing grooves on two sides opposite to the notch to provide overlapping stress fields that include transverse stress components.
Current test procedures require a minimum specimen thickness which cannot be satisfied in many cases. This serves to preclude use of miniature specimens. The advantage of the present invention is that specimens which are much smaller than those currently in use can be accurately tested. This enables testing of materials removed from in-service components in cases where it is not possible to remove enough material to meet current ASTM test requirements. The invention can also be used to provide additional nuclear pressure vessel surveillance data by cutting miniature specimens from the broken halves of full size charpy specimens. Another advantage of the method of the invention is that the method allows the modifying of the stress field such that mixed mode fraction in the transition region can be avoided or conservatively accounted for. The invention enables restricting fracture in the stress region to fixed mode fracture.
Another advantage of the present invention is that in-service stress fields can be simulated in the laboratory thereby providing data which is more representative of component performance.
This invention enables the determination of the DBTT of materials from miniature specimens; i.e. specimens noticeably smaller than prior conventional specimens in the materials testing field. More particularly, this invention relates to the testing of material specimens of a size, approximately one twentieth by volume more or less than thought necessary for valid determination of ductile-brittle transition temperature (DBTT) of solid material determined using Charpy specimens.